The humidification of air streams is well known using various techniques. In accordance with one technique as described in my U.S. Pat. No. 6,092,794, foggers which are supplied with pressurized air and water, create a fog that evaporates inside a stream of air that is led through ducts to areas that need to be humidified. The insertion of water fog can also be done, as is well known, by passing the air stream over a wet medium from which humidification is obtained.
Humidifiers for homes are on the market and which employ a so-called by-pass technique wherein a small percentage of a home's return air is by-passed through a passive wetted media and then re-introduced with water vapor from the media added into the supply side of the air flow. Typically, by-pass humidifiers are activated only during the time that the furnace is operating; thereby failing to provide adequate humidification during off-times of the furnace. The by-pass humidifiers also only humidify a small percentage of the total air flow that runs through the ductwork, thus reducing their effectiveness and their ability to attain set point levels. The by-pass humidifiers allow standing water to remain on both the wetted media and in their tanks, thereby encouraging unwanted microbial growth.
Humidification of air flowing through a duct is known in the art, and has been practiced with the use of steam vapor. For example, in one application of steam to humidify air, steam vapor is produced in a self contained steam generator from which steam vapor is fed through piping to a dispersion tube placed within a duct. The dispersion tube has small apertures through which the steam vapor is discharged into the air stream flowing through the duct. One or several dispersion tubes can be used. The steam vapor is typically under some pressure so that small apertures in the dispersion tube are used to discharge the steam vapor in the duct. Steam tends to require a substantial amount of energy to produce and its operating costs are, therefore, considerable particularly when generated with electric heaters. In addition steam tends to wet cool surfaces in the vicinity of its introduction into a duct and care must be taken to avoid the accumulation of water in the duct.
In U.S. Pat. No. 5,653,919 a humidification system is described in which a plurality of piezo-electric ultrasonic elements are located in a separate humidifying sub chamber which in turn is part of a larger chamber that is mounted on or inside a main air duct. The elements generate moisture laden columns of mist and water droplets which are permitted to pass through apertures in a plate for evaporation into a portion of an air stream available at an inlet port of the chamber. The humidified air is then introduced into a main air duct.
As described in the '919 patent the cross-sectional area of the humidifying chamber is made larger than that of the inlet port so as to slow the air stream speed and avoid carrying large un-evaporated droplets into the main air duct and thus enable large droplets to drain back into the reservoir of liquid located over the ultrasonic elements.
Various devices are described in the '919 patent to divert incoming air flow from directly impacting the liquid columns from the ultrasonic elements and retard the liquid mist they generate from exiting the humidifying chamber.
A similar system to that in the '919 patent is described in U.S. Pat. No. 5,702,648 wherein a stand alone humidifier unit is shown. This has an elongate zigzagged shaped humidification chamber whose effective length is at least seven feet in order to assure complete evaporation of the mist produced from ultrasonic mist elements.
In U.S. Pat. No. 4,986,937 an ultrasonic humidifier is shown, which is mounted in an air duct of a heating system. The water level covering the transducer is controlled with the use of a magnet that is mounted on a float valve to prevent actuation of the fogging system when the water level in a reservoir, in which the transducer is located, is too low.
In U.S. Pat. No. 6,511,050 a humidifier is described in which an ultrasonic transducer is mounted in water at the bottom of a vent stack and a fan is employed to drive air through the stack to its upper open end and thus deliver mist to a room. A vent stack is employed to control the water level over the transducer in the mist stack.
Other ultrasonic humidifiers are shown and described in U.S. Pat. Nos. 4,031,171; 4,752,423; 5,873,357; 5,832,176 and 6,244,576 B1.
Though these prior art devices may humidify air, they are not necessarily sufficient, both in an operational reliability and economic sense, for the humidification of residences, commercial and zoned facilities. Also, many of these devices generate a long plume of un-evaporated mist stream, thus requiring a long and large chamber to assure full evaporation and avoid condensation of water onto the walls of the duct.
What is needed, therefore, is a humidification apparatus that is reliable in achieving desired humidity levels and effective in producing humidification of a gas stream in a small space, avoids standing water and wetted media and accurately controls humidity levels. This can be particularly important in commercial sites such as semiconductor manufacturing plants and other manufacturing processes in which different types of gases need to be humidified with a particular liquid.